drm/i915: fix gpu hang vs. flip stall deadlocks

Since we've started to clean up pending flips when the gpu hangs in

commit 96a02917a0131e52efefde49c2784c0421d6c439
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Mon Feb 18 19:08:49 2013 +0200

    drm/i915: Finish page flips and update primary planes after a GPU reset

the gpu reset work now also grabs modeset locks. But since work items
on our private work queue are not allowed to do that due to the
flush_workqueue from the pageflip code this results in a neat
deadlock:

INFO: task kms_flip:14676 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kms_flip        D ffff88019283a5c0     0 14676  13344 0x00000004
 ffff88018e62dbf8 0000000000000046 ffff88013bdb12e0 ffff88018e62dfd8
 ffff88018e62dfd8 00000000001d3b00 ffff88019283a5c0 ffff88018ec21000
 ffff88018f693f00 ffff88018eece000 ffff88018e62dd60 ffff88018eece898
Call Trace:
 [<ffffffff8138ee7b>] schedule+0x60/0x62
 [<ffffffffa046c0dd>] intel_crtc_wait_for_pending_flips+0xb2/0x114 [i915]
 [<ffffffff81050ff4>] ? finish_wait+0x60/0x60
 [<ffffffffa0478041>] intel_crtc_set_config+0x7f3/0x81e [i915]
 [<ffffffffa031780a>] drm_mode_set_config_internal+0x4f/0xc6 [drm]
 [<ffffffffa0319cf3>] drm_mode_setcrtc+0x44d/0x4f9 [drm]
 [<ffffffff810e44da>] ? might_fault+0x38/0x86
 [<ffffffffa030d51f>] drm_ioctl+0x2f9/0x447 [drm]
 [<ffffffff8107a722>] ? trace_hardirqs_off+0xd/0xf
 [<ffffffffa03198a6>] ? drm_mode_setplane+0x343/0x343 [drm]
 [<ffffffff8112222f>] ? mntput_no_expire+0x3e/0x13d
 [<ffffffff81117f33>] vfs_ioctl+0x18/0x34
 [<ffffffff81118776>] do_vfs_ioctl+0x396/0x454
 [<ffffffff81396b37>] ? sysret_check+0x1b/0x56
 [<ffffffff81118886>] SyS_ioctl+0x52/0x7d
 [<ffffffff81396b12>] system_call_fastpath+0x16/0x1b
2 locks held by kms_flip/14676:
 #0:  (&dev->mode_config.mutex){+.+.+.}, at: [<ffffffffa0316545>] drm_modeset_lock_all+0x22/0x59 [drm]
 #1:  (&crtc->mutex){+.+.+.}, at: [<ffffffffa031656b>] drm_modeset_lock_all+0x48/0x59 [drm]
INFO: task kworker/u8:4:175 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/u8:4    D ffff88018de9a5c0     0   175      2 0x00000000
Workqueue: i915 i915_error_work_func [i915]
 ffff88018e37dc30 0000000000000046 ffff8801938ab8a0 ffff88018e37dfd8
 ffff88018e37dfd8 00000000001d3b00 ffff88018de9a5c0 ffff88018ec21018
 0000000000000246 ffff88018e37dca0 000000005a865a86 ffff88018de9a5c0
Call Trace:
 [<ffffffff8138ee7b>] schedule+0x60/0x62
 [<ffffffff8138f23d>] schedule_preempt_disabled+0x9/0xb
 [<ffffffff8138d0cd>] mutex_lock_nested+0x205/0x3b1
 [<ffffffffa0477094>] ? intel_display_handle_reset+0x7e/0xbd [i915]
 [<ffffffffa0477094>] ? intel_display_handle_reset+0x7e/0xbd [i915]
 [<ffffffffa0477094>] intel_display_handle_reset+0x7e/0xbd [i915]
 [<ffffffffa044e0a2>] i915_error_work_func+0x128/0x147 [i915]
 [<ffffffff8104a89a>] process_one_work+0x1d4/0x35a
 [<ffffffff8104a821>] ? process_one_work+0x15b/0x35a
 [<ffffffff8104b4a5>] worker_thread+0x144/0x1f0
 [<ffffffff8104b361>] ? rescuer_thread+0x275/0x275
 [<ffffffff8105076d>] kthread+0xac/0xb4
 [<ffffffff81059d30>] ? finish_task_switch+0x3b/0xc0
 [<ffffffff810506c1>] ? __kthread_parkme+0x60/0x60
 [<ffffffff81396a6c>] ret_from_fork+0x7c/0xb0
 [<ffffffff810506c1>] ? __kthread_parkme+0x60/0x60
3 locks held by kworker/u8:4/175:
 #0:  (i915){.+.+.+}, at: [<ffffffff8104a821>] process_one_work+0x15b/0x35a
 #1:  ((&dev_priv->gpu_error.work)){+.+.+.}, at: [<ffffffff8104a821>] process_one_work+0x15b/0x35a
 #2:  (&crtc->mutex){+.+.+.}, at: [<ffffffffa0477094>] intel_display_handle_reset+0x7e/0xbd [i915]

This blew up while running kms_flip/flip-vs-panning-vs-hang-interruptible
on one of my older machines.

Unfortunately (despite the proper lockdep annotations for
flush_workqueue) lockdep still doesn't detect this correctly, so we
need to rely on chance to discover these bugs.

Apply the usual bugfix and schedule the reset work on the system
workqueue to keep our own driver workqueue free of any modeset lock
grabbing.

Note that this is not a terribly serious regression since before the
offending commit we'd simply have stalled userspace forever due to
failing to abort all outstanding pageflips.

v2: Add a comment as requested by Chris.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -1660,7 +1660,13 @@ void i915_handle_error(struct drm_device *dev, bool wedged)
 			wake_up_all(&ring->irq_queue);
 	}
 
-	queue_work(dev_priv->wq, &dev_priv->gpu_error.work);
+	/*
+	 * Our reset work can grab modeset locks (since it needs to reset the
+	 * state of outstanding pagelips). Hence it must not be run on our own
+	 * dev-priv->wq work queue for otherwise the flush_work in the pageflip
+	 * code will deadlock.
+	 */
+	schedule_work(&dev_priv->gpu_error.work);
 }
 
 static void __always_unused i915_pageflip_stall_check(struct drm_device *dev, int pipe)